Lenz’s law: the induced current in a loop is in the direction that creates a magnetic field that opposes the change in magnetic flux through the area enclosed by the loop.
The induced current tends to keep the original magnetic flux through the circuit from changing.
Section 31.3
Lenz’ Law, Example
The conducting bar slides on the two fixed conducting rails.
The magnetic flux due to the external magnetic field through the enclosed area increases with time.
The induced current must produce a magnetic field out of the page.
The induced current must be counterclockwise.
If the bar moves in the opposite direction, the direction of the induced current will also be reversed.